Measurement of conductivity for most liquids or solids involves the use of electrodes which are placed into contact with the specimen of interest. A voltage is applied (usually AC) with resulting current measured and conductivity computed. In some cases, many electrodes are attached so that a kind of imaging is made possible, provided that conductivity varies spatially through the specimen. This latter condition is true for geological specimens and human tissue specimens.
An alternative is to generate eddy currents within the specimen through inductive coupling to an external coil. The eddy currents exist in proportion to the local conductivity of the material and can be detected in a number of ways. A recent technique in connection with attempting to measure conductivity of the human thorax, more specifically cardiac output, measures the additional electrical energy dissipated in the coil when placed near to the patient's body.
In spite of techniques employed to date, various technical issues prevent widespread use: not well known or understood; competing invasive methods; inadequate development of models to distinguish the variety of factors contributing to human tissue conductivity, making interpretation less straightforward; and use of either expensive or awkward instrumentation to measure coil related parameters such as complex impedance. Many devices use circuitry that permits the frequency of the exciting voltage to vary as a coil is placed adjacent its target material. Such an arrangement makes interpretation of measurements more confusing.